The document U.S. Pat. No. 5,163,736 describes an example of such a seat, which allows the back of the seat to be locked automatically when the vehicle is subjected to a violent accidental impact, which stops the seat from then pivoting freely under the effect of the impact when there is a chance of serious injury to the seat user, if this user is in the process of adjusting the inclination of the seat at the time of the accident.
In this known seat, the inertia safety device includes a balance weight which is normally kept in a neutral position by resilient means, and which is moved into a position where said balance weight controls the locking of the back when the vehicle is subject to acceleration or deceleration above a certain threshold.
This prior art seat has however the drawback that the kinetic energy of the balance weight, used on impact to overcome the force of the resilient means acting upon this balance weight and to lock the seat, is relatively low, given the light weight of the balance weight.
Because of this, on the one hand, it is relatively difficult to calibrate the inertia safety device, and on the other hand, the acceleration or deceleration threshold which triggers the locking of the seat back is quite high, so that said locking is not triggered for relatively slight accidental impacts to which the vehicle is subjected, whereas such impacts can nonetheless cause injuries to the seat passenger if the latter is in the process of adjusting the inclination of his/her seat back at the time of the accident.
Moreover, the operation of this type of inertia safety device is influenced by the inclination of the vehicle or of the seat part of the seat, since this inclination modifies the acceleration or deceleration threshold prompting movement of the balance weight and causing the back to lock.